This invention relates to flue gas conditioning, more particularly to a method for removing particulate and gaseous pollutants from flue gas using very fine mists of gas absorbing solution prior to discharge of the gas into atmosphere.
Generally, several methods of removing particulates from a gas stream are known. One method employs the use of electrostatic precipitators to precipitate electrically charged particles out of a gas stream. Electrostatic precipitators, however, require large quantities of energy and present safety problems.
Another method of removing pollutants uses impinging devices which provide barriers or packing to impinge on gas flow. As the gas moves through the system the particulates impact the obstructions and drop out of the gas stream. These devices are unsatisfactory, however, because the particulates collect in the packing and cause plugging and waste removal problems.
The most common method of treating flue gas to remove pollutants includes spraying water or mist onto the gas stream to suppress the particulate matter. Various types of these devices, known as wet scrubbers, are in wide use. Scrubbers have a major drawback in that their operation requires considerable energy. Furthermore, the conventional wet scrubber consists of pressure nozzles that produce droplets in the size range of several hundred microns. Droplets of that size are ineffective in capturing the respirable-size particulates contained in normal contaminates. The larger the size of the mist, the less effective surface area of treating liquid is provided. A surface area of a given amount of sprayed solution is: area=A=(3V)/(R), where V is the amount (volume) of solution sprayed and R is average diameter of the mists. Therefore, a finer (or smaller) mist increases the surface area of a given volume of solution sprayed.
Several prior art methods have been used to enhance the contact between the gas and the treating liquid. Generally these methods involve taking a large overall pressure drop of the flue gas. These methods include using a venturi with a pressure drop of 30 inches of water column or more or by bubbling the gas through a series of solution baths staged throughout the system. These methods are inefficient because the entire gas stream must be compressed to accomplish the contacting
U.S. Pat. No. 5,039,315 to Liao, et al. provides a method and apparatus for separating particulates from a gas stream. Other, relevant patents include U.S. Pat. Nos. 4,810,268, to Chambers et al; 4,401,644 to Teller; 4,345,916 to Richards et al; 4,269,812, 4,343,771, and 4,102,982 all to Edwards et al; 3,957,464, to Teller; 3,948,608 to Edwards et al; and 2,579,282, to Vicard.
All of the aforestated methods have another notable drawback. The methods may remove large particulate matter but are not as effective for the removal of small particulate matter, down to about five microns, or gaseous pollutants from the gas stream.